The present invention relates to a process for the separation of neptunium out of an organic phase, which is developed in the recovery of irradiated nuclear fuel and/or fertile material. In the recovery process, irradiated nuclear fuel and/or fertile material is dissolved in an aqueous, strong acid to form an aqueous phase or starting solution (feed solution) containing uranium-, plutonium- and neptunium ions, as well as /tritium in form of tritiated water and of fission products in ionic form. The uranium, plutonium and neptunion ions, as well as portions of the tritium in form of tritiated water and fission products in ionic form are then transferred from the aqueous phase into an organic phase comprised of an organic extraction agent dissolved in a diluent. This transfer of uranium and plutonium to the organic phase is known as coextraction. The organic phase is then subjected to a first wash step and a second wash step, with each wash step being a decontamination step, respectively, with respect to fission products.
Spent nuclear fuels, for example from Light Water Nuclear Reactors (LWR fuels), are recovered by the Purex process with the goal of recovering the valuable fissionable materials which are contained in them, namely, uranium and plutonium. The chemical and radio-chemical purity of the recovered uranium and plutonium end products must satisfy very high specifications. One of the radioactive impurities which must be removed from the uranium product with high efficiency is neptunium.
In LWR fuels, neptunium is developed mainly in the reaction chain .sup.238 U(n,2n) .sup.237 U(.beta.) .sup.237 Np. A burn-up spent fuel contains several hundred mg of Np per kg, but after the recovery, the uranium and plutonium products can be contaminated with at most .ltoreq.1 ppm or .ltoreq.1000 ppm of neptunium, respectively, to meet the very high specifications that have been set. That means for neptunium, during the entire process, decontamination factors of .gtoreq.500 or .gtoreq.50, respectively, must be attained in the uranium and plutonium purification.
The effective separation of neptunium from uranium and plutonium is above all made more difficult by the fact that neptunium can be present in different process solutions in two or even three coexisting valence states which differ greatly in their extractability, with the usual organic extraction agents such as, for example, with tributylphosphate (TBP) as extractant. Thus, neptunium (V) is practically not extractable with TBP, neptunium (IV) is quite weakly extractable with TBP, and neptunium (VI) has an extractability with TBP comparable with that of plutonium (IV). In many extraction operations of the Purex process, neptunium tends to be divided more or less evenly between the raffinate and the extract.
In the case when no attempt is made to influence the extractability of neptunium, a large portion of neptunium (to 99%) is extracted with the two elements in the first extraction operation of the Purex process, that is, in the coextraction of uranium and plutonium from the aqueous fertile material starting solution into the organic phase. See, W. Oschenfeld, F. Baumgartner, U. Bauder, H.-J. Bleyl, D. Ertel and G. Koch, "Experience with the Reprocessing of LWR, Pu Recycle, and FBR Fuel in the MILLI Facility", KfK 2558 (1977), Report of Kernforschungszentrum Karlsruhe GmbH. In the next step of the Purex process, that is, in the separation of plutonium from uranium, neptunium is divided into two comparable portions, which in each case accompany the uranium and the plutonium product. It would indeed be possible to shift the division of neptunium between these two products to the advantage of the uranium or plutonium product, but in order to do that, the concentration of nitric acid in the organic stream containing uranium and plutonium must be exactly controlled. However, in practice this is not always possible, and it must then be taken into consideration that the uranium product, as well as the plutonium product, is contaminated with neptunium and that the separation of neptunium occurs first in the following purification cycles for uranium and plutonium. There is then a decontamination factor of 200 to 300 or 20 to 30, respectively, to be attained.
It is also obvious that the problem would be most neatly solved if the neptunium were already separated from uranium and plutonium at the beginning of the extraction section of the Purex process. That is, the problem could be neatly solved if neptunium could be successfully sent into the raffinate stream during the coextraction of uranium and plutonium and the subsequent washes of the organic product. Neptunium is present in the starting solution (fuel solution) for the coextraction predominantly in the hexavalent form. A reduction agent must be found that reduces the extractable neptunium (VI) to nonextractable neptunium (V), but that does not also reduce the extractable plutonium (IV) to non-extractable plutonium (III). It is also important that neptunium (V) is not further reduced to partially extractable neptunium (IV). The fulfillment of all of these conditions is most difficult, because a rather high concentration of nitric acid (3 to 4.2 mole/liter) is employed in the aqueous phase in the coextraction of uranium and plutonium. The high acid concentration slows the reduction of neptunium (VI) to neptunium (V), and promotes not only a further reduction of neptunium to neptunium (IV), but also a disproportionation of neptunium (V) to neptunium (IV) and neptunium (VI). The resultant neptunium (IV) can then reduce plutonium (IV) to plutonium (III).
In the literature, only one possibility is described for neptunium to be transferred into the raffinate already during the coextraction of uranium and plutonium. See, W. L. Poe, A. W. Joyce and R. I. Martens, Ind. Eng. Chem., Process Des. Dev., Volume 3, 314+ (1964). The reduction agent used there, nitrite, indeed prevents the development of neptunium (IV) and plutonium (III), but it does not permit very effective separation of neptunium. In the Poe et al publication, no numerical data are to be found, but it can be understood from the text, and applicants' own experiences confirm, that a not inconsiderable portion of the neptunium is extracted with uranium and plutonium in the first extraction cycle and reaches the purification cycle. Thus, Poe et al indicate that neptunium escapes from the mixer-settler employed in the first extraction cycle when uranium and plutonium are coextracted into the organic phase, and that this escaped neptunium turns up in the second uranium cycle waste.